The present invention relates to color television cameras (hereinafter referred to as TV cameras) and more particularly to improvements in the white balance control system for TV cameras.
In the field of the automation of white balance control for TV cameras, a method has been proposed in which color temperature detecting light sensors having substantially the same red (R), green (G) and blue (B) imaging characteristics as the imaging devices of a TV camera and are capable of detecting a wider range of color temperatures than the angle of view of the phototaking lens of the TV camera and are arranged within or on the TV camera independent of the imaging devices therein whereby the changes in the color temperature of the light illuminating an object to be picked up is continuously detected by the light sensors and the gains of the TV camera color signals are continuously controlled in accordance with the detection results thereby automatically adjusting the white balance. This method has a great advantage in that even if the color temperature of the object illuminating light is changed due to the paning or tilting of the TV camera during the shooting, the white balance control unit is brought into operation and the desired follow-up adjustment is effected thereby making it possible to always shoot the object with the proper white balance adjustment without interrupting the shooting operation. This operating mode of the white balance control unit is referred to as an automatic follow-up mode in the description to follow.
A handy-type TV camera of the automatic follow-up white balance adjustment type is based on the assumption that if an object is picked up with a wide angle of view, it can be considered that the R, G and B color components included in the object are practically equal in amount. While it has been confirmed by experiments that this assumption is substantially satisfied in the ordinary photographing applications, the TV camera is used for a wide variety of purposes and the assumption cannot be satisfied in some special applications. A typical example of such special applications is a closeup photography. Let consider the case where a small insect resting on a wall is shot by the TV camera. In this case, if there is an extreme difference in extent between the photographing range corresponding to the angle of view of the phototaking lens and the color temperature detection range, the above-mentioned assumption is not satisfied so that if the color of the wall is red, for example, the white balance control unit is operated in such a manner that the gain of the R-component color signal is reduced and conversely the gain of the B-component color signal is increased to the maximum thereby producing a picture having an extremely bluish color tone.
Of course, the failure of the assumption is not limited to the above-mentioned close-up photography, that is, the assumption cannot be satisfied and the white balance will be destroyed generally in cases where any particular chromatic color occupies a large part of the detection range of the color temperature detecting light sensors in contrast to the color distribution within the photographing range.
In order to overcome the foregoing deficiencies, it is possible to operate the white balance control unit in an operating mode (hereinafter referred to as a memory mode) in which the outputs of the light sensors are stored and the gains of the color signals are maintained constant. In this case, while the difficulties of the previously mentioned automatic follow-up mode can be overcome, other difficulties, such as, the clearing of the stored contents of the memory upon the disconnection of the power source during the operation in the memory mode, the delay in response of the color temperature detecting sensors if the power source is connected while maintaining the memory mode and the delay in signal transmission due to other circuit elements are caused, thereby causing the white balance to deviate.